Many varied types of non-aqueous rechargeable lithium batteries are used commercially for consumer electronics applications. Typically, these batteries employ a lithium insertion compound as the active cathode material, a lithium compound of some sort (eg. pure lithium metal, lithium alloy, or the like) as the active anode material, and a non-aqueous electrolyte. An insertion compound is a material that can act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms).
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Currently available lithium ion batteries are high voltage systems based on LiCoO.sub.2 cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
The excellent reversibility of lithium insertion makes it possible for lithium ion batteries to achieve hundreds of battery cycles. Still, a gradual loss of lithium and/or buildup of impedance can occur upon such extended cycling for various reasons. This in turn typically results in a gradual loss in delivered capacity with cycle number. Researchers in the art have devoted substantial effort to reducing this loss in capacity. For instance, co-pending Canadian patent application serial number 2,150,877, filed Jun. 2, 1995, and titled `Use of P.sub.2 O.sub.5 in Non-aqueous Rechargeable Lithium Batteries` discloses a means for reducing this loss which involves exposing the electrolyte to P.sub.2 O.sub.5. However, P.sub.2 O.sub.5 shows at best only limited solubility in typical non-aqueous electrolytes and can be somewhat awkward to use in practice. Alternatives which are soluble may be more convenient, but it is unclear why such exposure is effective and hence what compounds might serve as effective alternatives.
Boron oxide (B.sub.2 O.sub.3) is a common chemical compound that is extensively used in the glass industry, and its properties are well known. B.sub.2 O.sub.3 has also been used in the lithium battery industry for a variety of reasons. In most cases, the B.sub.2 O.sub.3 is used as a precursor or reactant to prepare some other battery component. For instance, in Japanese published patent application 06-163046, M. Terasaki et al. use B.sub.2 O.sub.3 as a reactant to prepare a desired cathode compound. In Japanese published patent application 05-266880, Y. Mifuji et al. use B.sub.2 O.sub.3 as a reactant to prepare a desired anode compound. In Mater. Sci. Eng., B, B14(1), 121-6, 1992, C. Julien et al. use B.sub.2 O.sub.3 as a precursor to prepare solid or gel electrolytes for solid state lithium batteries.
In Japanese published patent application 07-142055, T. Maeda et al. show that lithium batteries can show improved stability characteristics to high temperature storage when using lithium transition metal oxide cathodes which contain B.sub.2 O.sub.3. However, there is no suggestion in the Maeda et al. application that improved battery characteristics might be obtained by having B.sub.2 O.sub.3 additive dissolved in the electrolyte or of possible ways of achieving this. Also, there is no suggestion in the Maeda et al. application that an advantage of employing a B.sub.2 O.sub.3 additive in the electrolyte could be to reduce the rate of capacity loss with cycling.